Due to a more and more complicated design principle of an intelligent mobile phone at present, various radio frequency devices, such as an antenna tuner in an antenna, a complicated antenna switch, a filter, a duplexer, a diplexer and a directional coupler, are required to be added between a power amplifier and an antenna, which result in an increased power loss between the power amplifier and the radio frequency devices.
In order to meet a requirement of the international standard (3GPP, 3rd Generation Partnership Project) on lowest power of an antenna interface, a manufacturer may generally enlarge output power of the power amplifier. For example, in a high-end intelligent mobile phone at present, highest output power of a GSM (Global System for Mobile Communication) power amplifier generally exceeds 36 dBm. A peak envelope current of the power amplifier may be as high as 2.5 A in a case of a load of 50 ohm, and may exceed 4 A in case of a severe mismatch of load. If the output power and a current of the power amplifier are not limited, the power amplifier may be damaged due to overheat, thereby resulting in malfunction of the mobile phone.